The present invention was conceived during development a stern drive to improve boating performance by modifying marine propulsion system configurations. Further, the invention relates to the addition of a multi-speed shift mechanism which does not require changing the original engine mounting position in the boat.
Existing single speed marine stern drives have only a single gear ratio or speed with ratios typically between 1:1 to 2.25:1. A gear reduction normally occurs between the engine crankshaft and the stern drive propellor shaft. Conventional stern drive units may also have forward, neutral, and reverse gear direction shifting capabilities.
Although such drives are capable of shifting directions, the gear ratio remains fixed. The propellor blade is also in a fixed position. This, in turn, limits performance. For example, if the system is configured for maximum power and speed at low speed, the drive will have less power and speed at high speed. Likewise, if the system is configured for maximum power and speed at high speed, the drive system will lose power and speed at lower speeds. A significant benefit of the present propulsion system is that the marine engine can be mounted farther back in the boat, usually 2″ to 12″ from the transom, without having to locate the transmission between the engine and the stern drive in the boat, thus providing more space inside the boat as well as affording better weight distribution and boat handling characteristics.
With conventional multi-speed marine systems, the stern drive may have forward, neutral and reverse gear direction shifting capabilities, but the gear ratio is a single, fixed ratio. The propellor blade is also in a fixed position. The gear ratio or speed change are accomplished by a transmission located between the engine and the stern drive. With existing transmissions, low gear and high gear speed ratios are available. The benefits of the propulsion system of the present invention is that multiple gear ratios are available so at low speed acceleration is improved and at high speed greater maximum or top speed of the boat is available.
With gear reductions usually being limited to 1.33:1 in low gear and 1:1 in high gear, the drawbacks are increased weight. When a transmission is added to existing systems, the engine mounting position is moved forward in the boat, away from the transom, usually from 12″ to 36″. This repositioning, in turn, can drastically effect boat handling characteristics, cause boat planing problems and limit available interior space. Accordingly, it is generally difficult to retrofit a multi-speed system into a single-speed designed boat due to the engine mounting position problems.